A senior aide to U.S. Republican presidential nominee Donald Trump signaled a possible shift in his hardline immigration policies on Sunday, saying his plans to deport 11 million people who are in the country illegally were under review.

Trump has put his vow to toughen the country's immigration policies at the center of his campaign. He has promised to carry out mass deportations and build a wall on the U.S.-Mexico border, proposals that critics have assailed as inhumane and too costly and unrealistic to achieve.

Trailing Democrat Hillary Clinton in opinion polls for the Nov. 8 election and struggling to broaden his support beyond the white working-class voters who have been his base of support, the New York businessman has reached out in recent days to black and Hispanic voters.

On Sunday, his new campaign manager, Kellyanne Conway, told CNN that Trump was committed to a "fair and humane" approach to those living in the country illegally.

"What he supports is to make sure we enforce the law, we are respectful of those Americans who are looking for well-paying jobs and that we are fair and humane to those who live among us in this country," Conway said on "State of the Union."

Pressed on whether Trump's plans would include a "deportation force" that the candidate previously pledged to set up, she replied: "To be determined."

Republican Senator Jeff Sessions, a close ally of Trump, told CBS's "Face the Nation" that Trump was still working through his plans for deportations should he win the White House.

"He's wrestling with how to do that. People that are here unlawfully, came into the country against our laws, are subject to being removed. That's just plain fact," the Alabama lawmaker said. "He's thinking that through."

Trump has also been rebuked by opponents for his proposal to impose a temporary "total and complete shutdown" of Muslims seeking to enter the country, later rolled back to focus on countries with "a proven history of terrorism."

Clinton has accused Trump of sowing divisiveness and said she would propose a path to citizenship for some migrants living in the United States illegally. The former secretary of state has said that militant groups like Islamic State have begun using Trump's proposed Muslim ban as a recruitment tool.

Campaign shake-up

Conway's comments came after Trump announced last week a major reshuffling of his campaign. Trump promoted Conway, who had been a senior adviser, to the role of campaign manager and hired Stephen Bannon, head of the Breitbart News website, as campaign chief executive.

The Trump campaign said on Friday that campaign chairman Paul Manafort was resigning.

The campaign's new leadership combines Bannon, a combative conservative, with Conway, a data-driven analyst who has been trying to broaden Trump's appeal to women and independent voters.

Trump's support has slumped in national polls in recent weeks and surveys in pivotal states such as Pennsylvania and New Hampshire have shown a widening lead for Clinton.

A Reuters/Ipsos survey released on Friday showed Clinton leading Trump nationally by 8 percentage points, 42 percent to 34 percent.

Trump vowed at a campaign rally in Fredericksburg, Virginia, on Saturday to return the Republican Party to the values of President Abraham Lincoln, who issued the Emancipation Proclamation and championed the 13th Amendment to the U.S. Constitution during the U.S. Civil War that led to the abolition of slavery in 1865.

Earlier on Saturday, Trump met with a group of Hispanic leaders as part of a new National Hispanic Advisory Council to the campaign.